The present invention relates to a hydraulic cement-based composition having a relatively long pot-life suitable for dip coating of the surface of steel- or iron-made articles, such as reinforcing rods for ferroconcrete, steel strings and sheaths for pre-stressed concrete, pipes and pipe joints made of steel or cast iron and the like. More particularly, the invention relates to a so-called polymer cement-based coating composition mainly composed of a hydraulic cement and capable of forming a protecting cured cement-based anti-corrosion coating layer to withstand the attack of a salty atmosphere with good adhesion between the substrate surface and the coating layer.
Some of steel- or iron-made articles are usually required, depending on the intended application thereof or the circumstances under which they are used, to have high corrosion resistance, to excellently withstand the influences of salty atmospheres and to have good adhesion with mortars and concretes in which they are embedded. Such requirements are usually satisfied by forming a protective coating layer on the surface of the substrate articles while such a protective coating is required to have particularly high adhesion to the substrate surface and flexibility not to be cracked by bending of the coated articles.
It is known that an organic-inorganic composite material or a so-called polymer cement composition composed of a hydraulic cement and an emulsion of a polymer can provide a very powerful protective coating layer on the surface of steel- and iron-made articles with excellent adhesion so that polymer cement compositions are highlighted in recent years as a heavy-duty anti-corrosion coating material on steel and iron. In contrast thereto, conventional anti-corrosion coating compositions for steel and iron are nothing more than a coating material to form a coating film on the steel surface merely to shield water and oxygen as the principal factors responsible for the formation of rust on the surface.
In comparison with the conventional anti-corrosion coating compositions on steel and iron, polymer cement compositions are very desirable materials capable of chemically forming a surface film of passive state expressed by the formula of gamma-ferrite .gamma.-Fe.sub.2 O.sub.3.nH.sub.2 O on the substrate surface by virtue of the strong alkalinity of the aqueous polymer cement composition having a pH of 12.5 or even higher. Disadvantageously, however, the passive-state surface film formed in this manner has a very small thickness of only 2 to 7 nm so that it is readily damaged under mechanical action and, in addition, the film is readily destroyed when the surface is brought into contact with anionic ions such as chlorine ions Cl.sub.- and sulfate ions SO.sub.4.sup.--. The polymer cement composition can provide a durable protective coating layer on the anti-corrosion passive-state surface film on steel and iron since the coating layer formed by curing a polymer cement composition is very tenacious and has excellent weatherability and mechanical strengths to provide physical or mechanical protection as being composed of the hydraulic cement and the polymer, optionally, together with aggregates.
In addition to the good adhesion to the surface of steel- or iron-made articles, polymer cement compositions composed of a hydraulic cement and a polymeric constituent added in the form of an aqueous emulsion are very versatile in respect of the rigidity or flexibility after curing by modifying the mixing ratio of the components. Namely, the cured coating layer may have increased rigidity by increasing the proportion of the hydraulic cement in the polymer cement composition while the cured coating layer can be imparted with good flexibility by decreasing the content of the hydraulic cement relative to the polymeric constituent in the composition. Such versatility of the coating layer relative to the rigidity or flexibility can never be obtained by any anti-corrosion protective coating compositions of other types.
One of the serious problems in the polymer cement compositions is the relatively short pot-life of the ready-prepared composition before the coating works. Namely, a polymer cement composition is prepared basically by compounding a hydraulic cement with an aqueous emulsion of a polymer as a vehicle so that the hydraulic curing reaction by hydration of the cement immediately starts when the hydraulic cement is blended with the aqueous emulsion and the consistency of the composition is gradually increased leading to the loss of the flowability of the composition within a relatively short time. No coating works can of course be undertaken with such a non-flowable composition which finally is solidified to be disposed by discarding. This problem of short pot-life practically prohibits the conventional process of coating works by dipping the steel-made articles in a bath of the polymer cement composition. Accordingly, it has been eagerly desired to develop a polymer cement composition greatly improved in respect of the pot-life after preparation of the composition without affecting the advantageous properties of the polymer cement compositions in general so that the coating works of steel- and iron-made articles can be performed by the most efficient and economical process of dip coating.
In this connection, the inventors have previously proposed an improved polymer cement composition capable of being stored with stability over a very long period of time of, for example, 2 years or even longer (see Japanese Patent Publication 57-54065). Characteristically, the storage stability of the polymer cement composition disclosed there can be increased by increasing the fineness of the particles of the hydraulic cement. Accordingly, it is a necessary step for the preparation of the polymer cement composition that the mixture of a hydraulic cement and the emulsion of a polymeric constituent must be subjected to prolonged mechanical grinding in a milling machine such as ball mills to have the cement particles suspended in the vehicle.
The storage stability of a polymer cement composition of 2 years or longer, however, is an overly extension of life more than practical significance. For example, a storage stability or pot-life of about 1 month would be sufficient from the standpoint of practical application of polymer cement compositions. Accordingly, the inventors have previously proposed an improved polymer cement composition which is prepared by compounding a colloid cement having a particle diameter reduced to 45 .mu.m or smaller beforehand so that the lengthy process of grinding after compounding can be omitted (see "Polymer Cement Coating Compositions", published by New Technology & Science Co., 1986, pages 178 to 187).
In the United States, a serious problem has been reported since 1970 that calcium chloride scattered on deposited snow with an object of accelerating thawing in winter causes heavy corrosion of steel rods embedded in reinforced conrete bodies by penetrating the concrete layer. Therefore, the road-managing authorities of the United States have to pay a great expense for the re-construction and maintenance of bridges and other structures under shortened durability due to corrosion of steel by the salty snow. A proposed solution for this problem is to provide steel rods with a coating layer of an epoxy resin by the techniques of electrostatic powder coating and the investigations for this process has been completed in about 1973 to enter the practical application of the process in the country and abroad (Cliften, Jr., H. F. Beeghly and R. G. Mathey, Non-metallic Coating for Concrete Reinforcing Bars, FH WA-RD-74-18, 1974). Although these epoxy-coated steel bars have no particular problems in respect of the resistance against salty atmosphere, they have several disadvantages including:
(1) increased cost of the steel bars of 1.5 to 1.85 times of that of uncoated ones; ones; PA0 (2) poor adhesion of the steel bars to mortar or concrete decreased by about 20%; and PA0 (3) danger of pinhole formation when the coated steel bars are worked by bending. PA0 (a) 100 parts by weight of a hydraulic cement; PA0 (b) from 10 to 36 parts by weight of an aqueous emulsion of a polymer capable of exhibiting good adhesion to the surface of steel and withstanding the attack of chloride ions, the amount of the emulsion being calculated as the content of the polymer therein; and PA0 (c) from 3 to 30 parts by weight of an aqueous emulsion of a hydrocarbon solvent containing from 0.5 to 2.0% by weight of a saccharide compound and a combination of from 0.8 to 2.0% by weight of a first non-ionic surface active agent soluble in the hydrocarbon solvent having an HLB value in the range from 10 to 14.5, from 0.8 to 2.0% by weight of a second non-ionic surface active agent soluble in water having an HLB value in the range from 15 to 18 and from 0.3 to 1.0% by weight of a cationic surface active agent soluble in water, the amount of the emulsion being calculated as the content of the hydrocarbon solvent and the amounts of the saccharide compound and the surface active agents each being based on the amount of the hydrocarbon solvent contained in emulsion.
It was also practiced in the United States since the beginning of this century to provide the inner walls of medium-diameter steel pipes for water service having a diameter of 750 mm or larger with a lining layer of cement mortar. A problem in this mortar lining was the poor adhesion of the lining layer to the steel surface and, so to say, the lining layer adhered to the steel surface by a mere mechanical arching effect and was susceptible to cracking and falling under external forces. Moreover, such a mortar lining layer was also poor in the water-shielding effect so that the lining layer of cement mortar on the inner walls of a water service pipe was neutralized within about 20 years to a depth of several centimeters from the surface requiring repair of the degradation.
Accordingly, it has been eagerly desired to develop a coating compositon capable of forming a coating layer on steel and iron surfaces having a very long durability of, for example, at least 50 years when a lining layer is formed on the inner surface of a pipe of steel or cast iron for water service and other steel-made structures such as pipes for crude oil mining by use of the coating composition.